1. Field of the Invention
The present invention relates to a method for data transmission, a write/read station for implementing the method, and a functional unit.
2. The Prior Art
In some known RFID (radio frequency identification) systems, the antenna for communication with the transponder is separated from a read/write station (reader), by means of a line that is generally configured as a coaxial cable. Such arrangements are mainly found where passive transponders are supposed to be read or written over relatively long distances, i.e. 0.7 to 1.5 meters, for example.
In such applications, other components are frequently necessary, in addition to the write/read station and the antenna. Thus, for example, so-called equalization devices or multiplexers are frequently required.
The task of an equalization device is, for example, to hold the antenna at a specific resonance frequency. On the other hand, the equalization device is allowed to perform the antenna equalization only if no communication is active between write/read station and transponder at the time.
Using a multiplexer, several antennas can be connected with a write/read station, and can be turned on one after the other.
Such and related devices are arranged somewhere between the write/read station and the antenna, in each instance, and must be controlled by the write/read station in suitable manner. According to the state of the art, control lines, which are implemented as additional connections between write/read station and external functional unit, were needed until now. This requirement has the disadvantage that an additional hardware expenditure is necessary and that additional installation effort is incurred in the installation of such an RFID system, as a result of which additional error sources can come about.
Furthermore, according to the state of the art, attempts have been made to control external functional units by means of simple pulses that interrupt the carrier signal for a defined period of time, or by means of direct voltages that are superimposed on the carrier signal. The disadvantage in the case of control by means of simple pulses or direct voltage, however, is that the write/read station needs additional hardware. Also, the control generally functions in only one direction, namely from the write/read station to the external functional unit. In addition, in the case of this method, no complex data or commands can be transmitted, which has the disadvantage, for example in the case of a multiplexer, that only sequential but not targeted relaying of the multiplexer outputs is possible. Furthermore, in the two methods stated, there is the risk that the emitted frequency spectrum of the RFID system will be changed by transmission of the pulses, in a manner that the limit values regulated by means of broadcasting regulations will be exceeded.